How
do you learn to fly at 13,000 miles per hour—a speed at which it would
take less than 12 minutes to get from New York to Los Angeles? Or, how
do you know whether a vehicle can maintain a long-duration flight while
experiencing temperatures in excess of 3,500 degrees Fahrenheit—hotter
than a blast furnace that can melt steel? And if you can fly, and
withstand the extreme heat, how do you know if the vehicle can be
controlled as it rips apart the air? How? You try it.

“Assumptions
about Mach 20 hypersonic flight were made from physics-based
computational models and simulations, wind tunnel testing, and data
collected from HTV-2’s first test flight—the first real data available
in this flight regime at Mach 20,” said Air Force Maj. Chris Schulz,
HTV-2 program manager who holds a doctorate in aerospace engineering.
“It’s time to conduct another flight test to validate our assumptions
and gain further insight into extremely high Mach regimes that we cannot
fully replicate on the ground.”

“Wind tunnels capture valuable, relevant hypersonic data and can operate
for relatively long durations up to around Mach 15. To replicate speeds
above Mach 15 generally requires special wind tunnels, called impulse
tunnels, which provide milliseconds or less of data per run,” Schulz
said. “To have captured the equivalent aerodynamic data from flight one
at only a scale representation on the ground would have required years,
tens of millions of dollars, and several hundred impulse tunnel tests.”
According to Schulz, impulse tunnel testing is required to create a
portion of Mach 20 relevant physics on the ground.

“And even then,” said Schulz, “we wouldn’t know exactly what to expect
based solely on the snapshots provided in ground testing. Only flight
testing reveals the harsh and uncertain reality.”

Approximately nine minutes into its first test flight in April 2010,
telemetry assets experienced a loss of signal from the HTV-2. The
vehicle’s onboard system detected a flight anomaly and engaged its
onboard safety system—prompting the vehicle to execute a controlled
descent into the ocean.

During its second test flight, “DARPA looks forward to conquering more
unknowns about long-duration hypersonic missions. We need to increase
our technical knowledge to support future hypersonic technology
development,” said Dave Neyland, director of DARPA’s Tactical Technology
Office. “We gained valuable data from the first flight, made some
adjustments based on the findings of an engineering review board to
improve this second flight, and now we’re ready to put all of that to
the test.”

For its second test flight, engineers
adjusted the vehicle’s center of gravity, decreased the angle of attack
flown, and will use the onboard reaction control system to augment the
vehicle flaps to maintain stability during flight operations.

A
technology demonstration and data-gathering platform, the HTV-2 is
packaged in a special capsule atop the launch-ready Minotaur IV Lite
rocket.

After the Minotaur rocket launches and nears orbit, HTV-2 will separate
and fly at a hypersonic glide trajectory within the earth’s atmosphere
Mach 20 speeds, approximately 13,000 miles per hour.

During the second flight test, more than 20 land, air, sea and space
test assets will collect data needed to improve predictions, through
modeling and simulation, of future hypersonic flight vehicle
performance—ultimately leading toward the capability of reaching
anywhere in the world in under an hour.